LONDON (Reuters) - Britain's competition authority is considering weighing into Hutchison Whampoa's (>> CK Hutchison Holdings Ltd) bid for Telefonica's (>> Telefonica SA) British mobile unit O2 UK, a deal that would cut the number of network operators to three and which was submitted for approval by the European Union's competition regulator last week.

The UK's Competition and Markets Authority (CMA) said on Wednesday it would seek views from the industry on whether the merger would substantially lessen competition in the British mobile market.

If the CMA decides it could damage competition, it could ask the European Commission, the EU regulator, for the right to refer it back to the CMA for an in-depth investigation.

The European Commission had said last week it would decide by Oct. 16 whether to clear the deal, which would make Hutchison's Three UK business the second biggest network operator behind EE and ahead of Vodafone (>> Vodafone Group plc).

Meanwhile the UK authorities are proceeding with the vetting of a separate $20 billion deal for BT (>> BT Group plc), owner of the country's biggest fixed line network, to acquire EE from its current owners Orange (>> ORANGE SA) and Deutsche Telekom.

Last week the prospects of the O2 and Hutchison merger getting approved were thrown into doubt after European Competition Commissioner Margrethe Vestager effectively blocked a plan by TeliaSonera (>> TeliaSonera AB) and Telenor (>> Telenor ASA) to combine forces in the Danish market because it would have reduced the number of operators there to three from four.

The CMA said given the short amount of time it had to make any request to the Commission, any representations should be received by Sept 24.

(Reporting by Paul Sandle; Editing by Greg Mahlich)